The public switched telephone network is made up of numerous local switching stations called xe2x80x9ccentral officesxe2x80x9d. The central offices are connected through a network of trunk lines, which carry the signals that represent a telephone call. Lines run from each central office to telephones in the local area around the central office. These lines are called xe2x80x9csubscriber linesxe2x80x9d because each user of the telephone network is called a subscriber. The trunk lines are xe2x80x9cswitched,xe2x80x9d which means that they are not associated with any subscriber. When a call is initiated, required trunk lines are dedicated to carry the call. When the call is finished, the trunk lines are reused to carry another call.
In contrast, each subscriber line has traditionally been dedicated to carrying the signals representing calls associated with a single user. Recently, loop concentrators and other architectural changes in the telephone network have allowed portions of the lines between a central office and some subscribers to be switched. However, there is still some portion of the subscriber line that is dedicated to carrying the calls for each subscriber.
As a result, there are tens of millions of subscriber lines in the US. There are also large numbers of subscriber lines in other countries throughout the world. Maintaining these subscriber lines represents a huge cost for the local telephone operating companies. To facilitate maintenance of subscriber lines, telephone operating companies employ automatic test equipment that can be attached to the network. One prior art product included a measurement unit that could be switched to each subscriber line at a central office. It would then measure the electrical characteristics on the subscriber line and, based on comparisons to expected values, indicate whether the line was faulty. Such a system allowed the telephone company to identify faulty subscriber lines so that a repairperson could be dispatched to repair the faulty line or lines.
Another prior art line test system makes xe2x80x9cnear endxe2x80x9d measurements, which means that it makes its measurements without attaching test equipment at the end of each subscriber line. Rather, the test equipment is located at a central point, thereby keeping the cost of test equipment low. Also, the measurements are generally non-interactive. Non-interactive measurements are made without the need for a person to inject a test signal or attach a test device at the far end of the subscriber line. Near end, non-interactive measurements are the least expensive and therefore the most desirable.
An additional prior art system also makes measurements on subscriber lines. Instead of simply reporting whether a line is faulty, this system gives an indication of where in the subscriber line the fault is located. Important elements of this system are described in U.S. Pat. No. 5,699,402, commonly assigned, which is hereby incorporated by reference.
This fault location information is important because it greatly reduces the cost of repairing the faulty line. Cost is reduced because, with more accurate information about the location of the fault, the right repair person with the right equipment can be sent to repair the fault. Less time is spent finding the fault so that the overall cost of repair is further reduced.
One type of fault that is particularly difficult to localize is a resistive fault. A resistive fault is a fault that can be modeled as an unintended resistive connection between one of the wires that makes up a subscriber line and some other point or points. The resistive connection might be between the two wires, called xe2x80x9ctipxe2x80x9d and xe2x80x9cring,xe2x80x9d that make up the subscriber line. The resistive connection might be between either tip or ring and some point that acts as a voltage source. Such faults might be caused by, for example, a cut or weak spot in the insulation around one of the wires.
Resistive faults are usually very difficult to localize using near end techniques. Resistive faults do not present a sharp transition in line impedance and are therefore hard to identify using time domain reflectometry. One possible way to localize a fault is by computing the resistance of the line between the measurement point and the fault. However, there has not heretofore been a suitable way to compute the line resistance because of the number of variables that might influence measurements taken at the near end. For example, the resistance between tip and ring, the resistance between tip or ring and a source of voltage and also the magnitude of that voltage will all influence the near end measurements and will be unknown values.
Because of these difficulties, the traditional way to localize a resistive fault is to use interactive techniques. A repairperson is dispatched and measures characteristics of the line at various points. By comparing measurements at various points, it is possible to tell whether a particular measurement was made before or after the resistive fault. In this way, the fault can be localized. Fault localization done interactively is slow and expensive.
It would be desirable to have a method wherein the number of faults that could be localized as well as the accuracy with which those faults could be localized was increased. In particular, it would be desirable to have a non-interactive technique to locate resistive faults.
With the foregoing background in mind, it is an object of the present invention to provide a method and apparatus to non-interactively test telephone networks and more specifically to localize faults such as resistive faults in the subscriber loop of the telephone network. The method and apparatus determine whether the fault is in the tip line or ring line of the subscriber loop. The existence of a fault voltage, if any, is also determined. The resistance of the tip line to ground and the resistance of the ring line to ground are measured. A fault resistance is calculated from this information, from which the location of the fault is determined.